In the fourth period of the periodic table, the subshells 4s,3d, and 4p are only filled. So number of orbitals available for filling = 1+5+3 = 9. So number of elements is 3*9 = 27, by considering the condition given in question (an orbital can hold 3 electrons).
Is this answer right. Because in my book answer is given as 48 by considering ymthe f subshell.